Cable Size Calculator: South Africa

Cable Sizing in South Africa: SANS 10142 Explained

Selecting the correct cable size is one of the most critical steps in any electrical installation. An undersized cable overheats, degrades insulation, creates a fire risk and will fail a Certificate of Compliance (CoC) inspection. Oversizing wastes money unnecessarily. SANS 10142, South Africa's wiring code, specifies exactly how cable sizing must be done.

There are two separate tests a cable must pass under SANS 10142, and both must be satisfied independently. This calculator checks both automatically.

The Two SANS 10142 Cable Sizing Checks

Check 1: Current Carrying Capacity

Every cable has a maximum continuous current it can carry safely without the insulation overheating. For standard copper PVC-insulated cables at 70°C operating temperature, these are the rated values used in South Africa:

Source: SANS 10142-1:2021 (Edition 3.1) Table 6.3(a) (current-carrying capacity) and Table 6.3(b) (voltage drop), multicore PVC-insulated unarmoured copper cable, 70°C conductor temperature, 30°C ambient. Current figures are for one two-core cable on a single-phase circuit, clipped direct (installation method 3) and in conduit on a wall or ceiling or in trunking (method 2); a three-core or four-core cable on a three-phase circuit carries less (2.5mm² clipped: 24A) and the calculator uses those figures for 400V. Conduit enclosed in an insulating wall (method 1) carries less again (2.5mm²: 18.5A).

Check 2: Voltage Drop

Even if a cable can carry the current safely, it must not drop too much voltage along its length. SANS 10142 sets the maximum voltage drop at 5% of the supply voltage for final circuits:

• 230V single phase: Maximum 11.5V drop
• 400V three phase: Maximum 20V drop

Why Installation Method Matters

A cable installed in conduit or enclosed trunking cannot dissipate heat as efficiently as one clipped to a surface or run in open air. This means the same cable has a lower current rating in conduit than when surface-mounted. The SANS 10142 derating for conduit is significant: a 2.5mm² cable drops from 27A (clipped direct) to 23A (in conduit on a wall or in trunking), and to 18.5A where the conduit is enclosed in an insulating wall (SANS 10142-1 Table 6.3(a), installation methods 3, 2 and 1). The calculator uses the conduit-on-a-wall figure. Failing to apply this derating results in overloaded cables that overheat and degrade.

Common SA Circuit Examples

These are typical cable sizes used in South African residential installations. Always verify with a registered electrician, as actual installations depend on run length, grouping and ambient temperature.

• Lighting circuits: 1.5mm² (up to 20m runs), 2.5mm² for longer runs
• 15A plug circuits: 2.5mm² (short runs), 4mm² (over 25m)
• Geyser 3kW: 2.5mm² at standard 30°C conditions, 4mm² in a hot roof space (50–60°C, common in SA)
• Electric stove 6–9kW: 6mm² minimum, 10mm² for longer runs
• Air conditioner 3–5kW: 2.5mm² (short runs), 4mm² (over 15m)
• DB board sub-mains: 10mm²–25mm² depending on total load
• Inverter / solar feed: 10mm²–25mm² depending on inverter size and run length

Ambient Temperature Derating, Critical in South African Conditions

The current-carrying capacity values in SANS 10142 assume an ambient temperature of 30°C. In South African conditions, roof spaces in summer, industrial environments and direct-sun surface installations, ambient temperatures frequently exceed 40°C, which meaningfully reduces cable capacity. SANS 10142-1 Table 6.10 gives these ambient temperature correction factors for general-purpose 70°C PVC-insulated cable: 0.94 at 35°C, 0.87 at 40°C, 0.79 at 45°C, 0.71 at 50°C, 0.61 at 55°C and 0.50 at 60°C. A 2.5mm² cable rated at 27A clipped direct at 30°C is limited to about 19.2A in a 50°C roof space and to 13.5A at 60°C, only just above a standard 3kW geyser's 13A draw; in conduit (23A) it falls to 11.5A at 60°C, below that draw. Use the calculator's Ambient Temperature input above to apply the correct factor directly, particularly for geyser circuits and solar panel DC cabling installed in enclosed roof voids.

Grouping Derating, When Cables Run Together

When multiple current-carrying cables are bundled or run in close contact, the combined heat from adjacent cables reduces each cable’s individual capacity. SANS 10142-1 Table 6.14 gives these grouping correction factors for cables enclosed in conduit or trunking, or bunched and clipped direct to a non-metallic surface: 0.80 for 2 circuits, 0.70 for 3, 0.65 for 4, 0.60 for 5 and 0.57 for 6, falling further for larger groups (0.52 for 8, 0.48 for 10). This is particularly relevant in South African installations where multiple circuits are routed in a single conduit or cable tray between the DB board and distribution points. A 2.5mm² cable carrying 20A in a conduit is safely loaded when run alone but may be overloaded when sharing the conduit with two other loaded circuits. Use the calculator's Cables Grouped Together input above to apply the correct factor, alongside ambient temperature, since both derating effects combine and neither substitutes for the other.

A Worked Example: Why Roof-Space Heat Changes the Answer

A standard 3kW geyser draws about 13A at 230V. For a 35m run in conduit (a larger home, geyser well away from the DB board), a 2.5mm² cable at the standard 30°C reference has a 23A capacity, comfortably enough, and passes voltage drop too (8.2V against the 11.5V limit). But geyser cabling almost always runs through a roof space, and South African roof spaces regularly exceed 50–60°C in summer. At 60°C the ambient correction factor is 0.50, cutting that 23A rating to 11.5A, below the geyser's 13A draw and no longer compliant. A 4mm² cable, rated 30A in conduit, derates to 15A at the same 60°C, covering the load, and drops 5.0V over the 35m. This is why many SA installers default straight to 4mm² for geyser circuits: not because 2.5mm² fails under standard test conditions, but because it can fail once real roof-space heat is properly accounted for. Run this exact scenario through the calculator above (3000W, 230V, 35m, in conduit) at 30°C and then again at 60°C to see the recommendation change from 2.5mm² to 4mm². If it is clipped direct instead of in conduit, a 2.5mm² cable is rated 27A and still just passes at 60°C (13.5A), with 0.5A to spare, so any further derating such as a second cable alongside it tips it over.